This invention relates to a method of forming a fluorescent screen of a color picture tube, more particularly a method of forming a fluorescent screen of a color picture tube wherein powders of phosphors are coated on the inner surface of the face plate or panel of a color picture tube by so-called dry process.
The dry process was developed to replace conventional slurry process. According to the dry process, a photosensitive composition consisting essentially of an aromatic diazonium salt is used. When locally exposed to light, a film of this composition manifests a difference in the powder accepting capability between a portion exposed to light and a portion not exposed to light.
According to this method, a film of photosensitive substance is coated on the inner surface of the face plate, and the portions of the film at which phosphors of predetermined colors are to be coated are exposed to light through a color selection electrode such as a shadow mask to utilize a photoreaction created therein for the purpose of coating the phosphors. More particularly, a film of a photosensitive composition containing a diazonium salt as a major ingredient is coated on the inner surface of the face plate and portions of the film at which the phorphors are to be coated are exposed to light. Then zinc chloride formed in the exposed portions by photoreaction absorbs moisture in the atmosphere to become sticky so that when a powder of phosphors for emitting desired fluorescent colors is blasted onto the inner surface of the face plate, the phosphor powder will adhere only to the portions of the photosensitive composition film that have become sticky. After performing blasting and sticky coating of respective phosphors of three colors, the surface of the photosensitive composition film is treated with ammonia gas to render the portions of the photosensitive composition film to which the phosphor powder has been adhered to be insoluble in water, thereby fixing these portions. Then, the surface remainders are washed away with an organic solvent thus forming a fluorescent screen. When compared with the slurry type method in which phosphors of respective colors are formed as photosensitive slurried which are sequentially coated, exposed, washed with water and developed, this dry process is advantageous in that only one coating step of the photosensitive composition film is sufficient, that is it is possible to blast the phosphors in the form of a powder, and that surplus phosphor powders that had not adhered can be blown away with air spray for recovery purpose, thus increasing the efficiency of utilization of the phosphors. Such dry method is disclosed, for example, in Japanese Preliminary Publication of patent No. 126,861 of 1978 published on Nov. 6, 1978 invented by Saburo Nonogaki et al, and corresponding to U.S. patent application Ser. No. 895,372 filed on Apr. 11, 1978, now U.S. Pat. No. 4,273,842.
Irrespective of the advantage described above, in the dry process there is a problem that the powder of phosphors applied in the succeeding step tends to adhere to the peripheries of the phosphor powders coated in the preceding step. More particularly, in the step of forming the phosphor film of a first color in which portions to be exposed are caused to absorb moisture in the air to become adhesive and the phosphor powder of the first color is caused to adhere to such moisture adsorbed peripheries. However, since the photosensitive film itself is water soluble, the moisture is adsorbed only in the peripheral portion of the film to be exposed to cause the peripheral portion to become adhesive. Accordingly, after light exposure, when the phosphor powder of a second phosphor is coated on the exposed portion for the purpose of forming the phosphor film of the second color, the phosphor powder of the second color would adhere only to the peripheries of the portions coated with the phosphor of the first color. In the same manner, in the step of forming the phosphor film of a third color, the phosphor powder of the third color would adhere to the peripheries of the portions coated with the first and second colors. Ooze of the moisture causes so-called blurring or fogs in which powders of the phosphor adhere to not exposed portions, thereby degrading the color purity of the color picture tube.